Presidenfs Address. 
53 
year apart, may be considered as viewed from two points in space 
separated by this length, and it only requires lapse of time in order 
to increase the length to an almost indefinite extent. 
The great scheme for the photographic mapping of the heavens 
at present being carried out on extensive- scale by means of the 
co-operative efforts of the leading observatories of the world will shortly 
furnish a highly accurate delineation of the skies as seen at the 
commencement of the twentieth century. This alone has called for 
concentrated effort extending over some twelve years at least, while it 
would even now scarcely be safe to say that another ten years will see 
its completion. An immediate repetition is scarcely to be contemplated, 
though a subsequent repetition at some future epoch, which may be 
agreed on by astronomers, forms an essential part of the programme as 
originally introduced. 
When this scheme is completed in its entirety very ample data will be 
available for the discussion of stellar distribution by the methods I have 
suggested to you. 
In the meantime, however, in such tentative attempts as have been 
made to fathom the secrets of the universe by means of the study of 
stellar proper motion, it has been necessary to rely on early recorded 
exact observations. It will be clear from what I have already explained 
to you, that it is the earliest reliable records in comparison with the most 
up-to-date available which will yield the greatest length of base-line 
and consequently the most trustworthy results. 
For this reason the majority of the discussions hitherto attempted 
have been based on the catalogue of Bradley, dependent on observations 
made by him at Greenwich between the years 1750-62. 
This catalogue contains the places of some 3,000 stars observed with a 
precision far surpassing any similar previous observations and comparing 
favourably with the best modern catalogues. The stars selected by 
Bradley are fairly uniformly distributed over the portions of the sky 
accessible to him, viz., from the North Pole to 30° South of the Equator. 
Unfortunately no early catalogue of stars of even approximately 
similar precision exists for the remaining region of the sky between 
30° S. Decl. and the South Pole, and the absence of exact knowledge 
of these regions for the earlier epochs has always hampered these 
discussions. 
The discussions I refer to have generally had as their immediate 
objective — 
1. The determination of the precessional constant, i.e., the annual 
amount by which the earth's axis of rotation changes its position in 
space, and — 
2. The determination of the speed of the solar motion and the position 
